1. Field of the Invention
The present invention relates to a powdered fuel cell and, in particular, to a powdered fuel cell having low cost, high energy density, high efficiency and the capability for reducing weight and energy use.
2. Descriptions of the Prior Art
The capability of the cell for converting chemical energy to electrical energy and conversely has been well known. However, in manufacturing of cells for many applications, e.g., electric vehicle, certain factors need be taken into consideration such as high energy density, high energy conversion efficiency, low cost, long life cycle, safety, convenience and low effect on the environment. In general, the compositional weight and volume of the typical cell would be increased due to those factors. Therefore, it is very difficult to construct an operationable, safe and convenient commercialized practical cell.
The fuel cell is an electrochemical apparatus, wherein a portion of the energy from the chemical reaction is converted to direct-current electrical energy directly. The direct conversion of energy to direct-current electrical energy dispels the demand for conversion of energy to heat and, therefore, avoids resulting in the limitation set by the effect of the Carnot cycle. In absence of effect of the Carnot cycle, theoretically the fuel cell has an efficiency higher than the traditional energy generating apparatus (e.g., internal combustion engine) by 2 to 3 times.
Fuel cell are classified according to the fuel:
(a) Gas fuel cell (hydrogen, carbon monoxide, gas hydrocarbon);
(b) Liquid fuel cell (alcohol, aldehyde, bydhazine, hydrocarbon, chemical compound);
(c) Solid fuel cell (coal, charcoal, coke, metal flake).
Because of the energy shortage and the green house effect in recent years, and by the demand for the high performance of clean energy or urgently for independent power source for transport and electricity load, research on new electrochemical batteries has been pushed through significantly. Typical fuel cells utilize a polymeric membrane for the ions of electrolytes, which is conventionally the polymeric proton exchange membrane (PEM), as an electrolyte ion exchange membrane. Ion membrane is placed between anode and cathode, which are gas diffusion electrodes exposed to the respective reducing agent and oxidizing agent gas for the reaction thereof.
Thus, when electrochemical reaction occurs, each contact between those two contacts (three-phase interfaces) is an interface between the electrolyte polymer and the reactant gas for the electrodes. For example, when the oxidizing agent gas is oxygen and the reducing agent gas is hydrogen, hydrogen is supplied to the anode and oxygen is supplied to cathode.
The overall chemical reaction is 2H2+O2→2H2O, for which the electrochemical reactions occurring at a precious metal catalyst are shown as follows:Reaction at anode: 2H2→4H++4e− E1/2=0.828V;Reaction at cathode: O2+4H++4e−→2H2O E1/2=0.401V.
This relates to the well-known hydrogen fuel cell available commercially. The technology is mature, whereas it cannot replace current internal combustion engines for the following disadvantages:
1. High cost: high-price ion membrane is needed for use; high-price precious metals, e.g., platinum or ruthenium, are needed for the electrode catalysts.
2. Low energy density: theoretically one kilogram of hydrogen storage material has hydrogen content of about 1.5% by weight, as being expensive; the energy density is even lower when hyperbaric hydrogen or liquid hydrogen is used alternatively.
3. Safety: it is very dangerous since hydrogen burns fast and is flammable and explosible.
4. Low efficiency of energy resource: it has efficiency of 30-60% only.
5. Convenience: Appropriate thermal management is necessary during the processes of storage of and release of hydrogen, no matter that hyperbaric hydrogen, liquid hydrogen or metal hydrogen storage material is used and, therefore, it is inferior to gasoline having advantages such as fastness, convenience and replenishment depending on the amount used.
The five aspects described above are the most difficult in practicing the fuel cell. Moreover, it is difficult to make high-purity hydrogen (impure hydrogen would poison the electrodes) so that the price of hydrogen is many times higher that that of gasoline.
In order to mend such problems, scientists have proposed such a chemical compound as sodium borohyride for liquid fuel cell, wherein sodium borohyride has reaction:Reaction at anode: BH4−+8OH−→BO2−+6H2O+8e− E1/2=1.24V;Reaction at cathode: 2O2+4H2O+8e−→8OH− E1/2=0.401V;Overall chemical reaction: BH4−+2O2→BO2−+2H2O E0=1.64V.
This compound, which is soluble in strong base at the positive electrode and in strong acid at the negative electrode, may settle the problem of storage and energy density, but it is associated with incomplete consumption of energy with 10-20% residual that cannot convert, like ordinary batteries. Since the compound tends to be affected by the air and then oxidized and contaminated to release hydrogen, expensive ion membrane (e.g., PEM) and noble metals should be employed to prevent self-discharge. In the aspect of safety, during the replenishment of fuel the compound may leak out and contaminate the environment or may damage the users due to the strong acid or base replenished, which would be dangerous if leaking out. Technology for the compound in this aspect has not yet matured.
Additionally, a metal-air fuel cell has been proposed that electrochemically couples a reactive metal electrode to an air electrode through a suitable electrolyte. As well known in the art, the electrolyte may be typical caustic liquid or sodium chloride, which is ionically conductive but not electrically conductive. Therefore, the air cathode is formed into a flake shape and has opposite surfaces respectively exposed to an electrolyte of the cell and the atmosphere, wherein the oxygen in the atmosphere may decompose (during the action of the cell) and the metal of the anode may undergo oxidization so that an appropriate current is provided through the outer circuit connecting the anode to the cathode and thereby the electrically conductive devices used in the outer circuit are combined certainly.
The zinc metal-air fuel cell has equations thereof as follows:Electric reaction at cathode: ½O2+H2O+2e−→2OH− 0.401V;Electric reaction at anode: Zn+2OH−→ZnO+H2O+2e− 1.245V;Theoretical generation of electricity: ½O2+Zn→ZnO 1.645V.
The actual open-circuit voltage is about 1.5 V. The metal material that can be used to be oxidized includes zinc, iron, magnesium, calcium, tin, aluminum, lithium or alloys thereof, and it can be present as metal or the oxide thereof.
The currently employed commercial metal-air cells have huge volume and low energy density, require replacing the whole cell or charging to replenish with electricity, and consume energy incompletely with 10-20% residual unoxidized, so that the electricity would be wasted and the efficiency influenced very inconveniently. Moreover, metal-air cells have another handicap with respect to air (catalyst) and heat management and to the associated intrinsic volume expansion of the metal. For example, the zinc electrode expands when the metal zinc oxidizes to become zinc oxide and zinc hydroxide with a resultant change of volume since zinc powder has a specific weight of 7.14 while zinc oxide has a specific weight of 5.06; due to the difference in specific weights, the volume of zinc powder would expand after oxidization and such change of volume would result in an overflow of the electrolyte and a bend of the anode. Metal-air cells have yet another handicap with respect to cell failing caused by deteriorated anodes that leads to uneven discharge and thereby decreases the power output. Thus, it is very hard to replace gasoline with metal-air cells though they are inexpensive.
FIGS. 1 and 2 show a conventional metal flake fuel cell and a conventional pellet fuel cell, respectively, which would suffer such problems after assembling in series as wind flow management, heat management, expansion of metal, and overflow and leakage of electrolyte and have derived numerous patents in association, e.g., U.S. patent application Ser. Nos. 60/340,592, 60/380,048, 60/387,355, 60/285,850, 60/384,547, 60/384,550, 60/391,860, 60/340,592, 60/389,821, 60/386,121, 60/326,432, 60/346,128, 09/805,419, 09/621,836, 09/893,163, 60/288,675, 60/292,237, 09/258,573, 09/584,875, 60/301,558, 60/312,659, 09/695,698, 09/695,699, 60/290,945, 60/286,199, 09/594,649, 09/414,874, 60/275,786, 09/695,697, 60/358,229, 60/274,337, 09/827,982, 60/344,546, 60/324,867, 60/340,697, 60/298,537, 60/295,634, 60/267,819, 60/286,198, 60/263,174, 60/270,952, 60/267,933,60/261,126, all mentioning the foregoing problems. The present invention may settle all these problems rather practically.